mx1986 Posted October 12, 2015 Report Share Posted October 12, 2015 I've got a 421 cub with 39mm pwks with a .109 main timing a +8 with 17cc shelf domes. I've got a digatron on the bike and still only getting to just under 1000 degrees with the pj's closed. I'm just curious to how small some of your guys main jets are to achieve that 1100 to 1300 egts. My plugs are still not even showing a heat mark. I went down from a .116 to a .109 and only picked up 70 degrees egts. Quote Link to comment Share on other sites More sharing options...
mx1986 Posted October 12, 2015 Author Report Share Posted October 12, 2015 Also at about 900 feet a over sea level Quote Link to comment Share on other sites More sharing options...
n2otoofast4u Posted October 12, 2015 Report Share Posted October 12, 2015 How big are the dumps? IMO put more timing to it also! Quote Link to comment Share on other sites More sharing options...
mx1986 Posted October 12, 2015 Author Report Share Posted October 12, 2015 .120 dumps Quote Link to comment Share on other sites More sharing options...
J-Madd Posted October 13, 2015 Report Share Posted October 13, 2015 Those are some small domes. No reason to need a main that small AND pjs closed. Even with 0.120" dumps you should have heat on the plugs. Assuming EGN or similar needle. Making the mains much smaller than the dump probably won't do much. The motors not getting loaded or something. Are these 300 ft runs? I agree with adding some timing but something else is up. What pipes? What's the compression (psi)? Is it ported? Quote Link to comment Share on other sites More sharing options...
Abe Posted October 13, 2015 Report Share Posted October 13, 2015 My buddy is having similar issues. He also has 39pwk's .120 dumps .110 mains. Carbs ran fine on a 4mil cub with pj's 3 turns out on average at our local track. He now has a 10 mil cub at 195psi. It will rev fine in neutral but once you leave the line under load it won't clear out unless the pj's are closed down to about 1 turn out. I'm not familiar with alcohol so it scares me having the pj that far closed especially knowing the 4 mil ran them opened up much more. both motors had BB shearer oof pipes. any words of wisdom would be appreciated Quote Link to comment Share on other sites More sharing options...
trickedcarbine Posted October 13, 2015 Report Share Posted October 13, 2015 Shelf domes, like pro design domes? Quote Link to comment Share on other sites More sharing options...
Abe Posted October 13, 2015 Report Share Posted October 13, 2015 I didn't assemble motor to know squish specs but has 18cc domes from cascade and a .020 base gasket. Would too loose of squish cause this issue if domes were setup for .012 base gasket ? both motors set at 8* advance as well Quote Link to comment Share on other sites More sharing options...
J-Madd Posted October 13, 2015 Report Share Posted October 13, 2015 In general longer stroke = smaller jet needed. Quote Link to comment Share on other sites More sharing options...
n2otoofast4u Posted October 13, 2015 Report Share Posted October 13, 2015 4 cubs always seem to be thirsty. At 195psi I'd be looking to have some dome work done. I know nothing. Quote Link to comment Share on other sites More sharing options...
sleeper06 Posted October 13, 2015 Report Share Posted October 13, 2015 I just dialed in a 421 cub with 39 pwk alky carbs . I ended up with a 60 pilot .122 dump/ main egn needle 2nd clip , power jets drilled to .096 4 turns out so far on pj heated the plugs up, every motor will want a lil diffrent set up start 6 turns out and squeeze it down slow Quote Link to comment Share on other sites More sharing options...
mx1986 Posted October 13, 2015 Author Report Share Posted October 13, 2015 Motor has rdz pipes and the ports have been cleaned up. Not sure on the needle will have to check it. The carbs came off a rmk 700 snowmobile that I drilled out the dumps and installed a powerjet setup for pulse pump. The domes are 17cc bought off ebay probably trinity can't remember. Quote Link to comment Share on other sites More sharing options...
sleeper06 Posted October 13, 2015 Report Share Posted October 13, 2015 Drill pj out to .096 start with 120 dump / main with 55 pilot , egn needle in the middle , start 6 turns out and see if u get a good burn on the plug Quote Link to comment Share on other sites More sharing options...
J-Madd Posted October 13, 2015 Report Share Posted October 13, 2015 If the needles aren't the issue, the pump could be feeding the engine and hiding what should be a lean carburetor setup. Quote Link to comment Share on other sites More sharing options...
mx1986 Posted October 13, 2015 Author Report Share Posted October 13, 2015 you think the pump is pushing past fuel? Quote Link to comment Share on other sites More sharing options...
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